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An Overview of Yellowstone GeologicHistory

Introduction

Yellowstone National Park —the nation's first national park, established in 1872—occupies 2.2million acres in northwestern Wyoming and southwestern Montana. Located along thecontinental divide within the Middle Rocky Mountains, Yellowstone is on a high plateauaveraging 8,000 feet in elevation. The mountain ranges that encircle Yellowstone vary from10,000 ft to nearly 14,000 ft, and include the Madison Range to the west; the Gallatin andBeartooth Mountains to the north; the Absaroka Mountains on the eastern border; and the TetonRange, within Grand Teton National Park , at the southern end. Yellowstone National Park contains the headwaters of two well known rivers, the Yellowstone River and the Snake River.

The Ecosystem

Yellowstone forms the core of theGreater Yellowstone Ecosystem (GYE)which comprises approximately 18million acres of land encompassingportions of Wyoming, Montana, andIdaho. The GYE includes three nationalwildlife refuges, five national forests,and two national parks. This area is thelast intact contiguous temperateecosystem in the world! Elevation, river

systems, mountain topography, wildliferanges, characteristic flora and fauna,and human-use patterns are uniqueinside the GYE when compared to thesurrounding lands. The GYE stillcontains nearly all of the livingorganisms found in pre-Columbiantimes, though not in the same numbers.

Because of the tremendous biodiversityprotected within Yellowstone, the Park

was declared an International BiosphereReserve in 1976 and a World HeritageSite in 1978. While wolves and grizzlybears are spectacular examples of theimportance of protecting park resources,it is Yellowstone’s thermal features thatled to its initial protection.

Map of the Greater YellowstoneEcosystem.( Lighter colored borders indicate the approximateecosystem boundary)

Biodiversity—from moose to microbes—has been shaped and formed by the Park's underlyinggeology. Processes of mountain uplift, glaciation, and physical weathering have shaped the

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landscape, and have influenced both mountain and forest geography. In recent geologic time,violent volcanic activity has most prevalently shaped Yellowstone’s topography.

The unique ecosystem, biodiversity and geologic features are intimately related to the geologichistory of Yellowstone National Park. Glaciation, tectonic activity and, most importantly,volcanic eruptions, have shaped and influence the landscape and life within the park for millionsof years.

What is the source of the heat that derives the Yellowstone thermal and volcanic activity?

Geologists theorize that a hotspot in the upper mantle of the earth's crust is responsible. Similarhot spot activity resulted in the formation of the Hawaiian Island Chain.

Interpreting the Landscape: Recent and Ongoing Geology of Grand Teton and Yellowstone National Parks by John M. Good and Ken L.

Pierce, 1996, Grand Teton Natural History Association

Hotspots

On the left (top) , the pie shapedslice through the Earth shows aninterpretation of the Yellowstoneand Hawaii plumes and hotspots.The plates of plate tectonicsconsist of the crust bound to thesolid uppermost mantle. (Platesshown here at 5 times their actualthickness.) The plates glide uponthe next lower, more squishy partof the mantle called the

asthenosphere which is hotter andmore plastic. Mantle plumes areshown originating at the core-mantle boundary and rising to thebase of the plates. At spreadingcenters, new crust from morefluid, mantle (athesnosphere), iscreated.

At plate boundaries (diagramleft), slabs of cold lithosphere are

subducted and sink deep into themantle. Black dots representhotspots. Arrows show theapparent movement of thehotspots as plates drift over them.Most hotspots are beneath oceans;however the Yellowstone hotspotis under a continent.

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Windows into the Earth: the geologic story of Yellowstone and Grand Teton National Parts by Robert B. Smith and Lee J. Siegel,2000 Oxford University Press.

The diagram (left) depicts moltenrock, or magma, rising inconvection cells like water boilingin a pot. The left side showstraditional view of hot spots as aplume of magma rising from theEarth's core. The right side showsthe new theory that Yellowstonehotspot originated at a muchshallower depth.

At present the North Americantectonic plate is moving about 1.8inches centimeters per year in asouthwesterly direction. The slowsteady progression of the plateacross the magma plume has left itsmark on the landscape; as evidencedby the flat-lying basalt volcanicflows across the Snake River Plainof southern Idaho. The plume worksmuch like a blowtorch underneath asheet of metal, causing melting andbubbling of the metal at the surface.Volcanic activity between the Idahocities of Twin Falls and Idaho Fallshas occurred for millions of years.Craters of the Moon NationalMonument is a recent example of this pattern, with YellowstoneNational Park acting as the currentlocation of activity.

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Yellowstone Volcanic History

Volcanic activity in Yellowstone is, in part, a fairly recent phenomenon, geologically. Over thelast 2 million years, three gigantic caldera eruptions have left their mark on the landscape andhave influenced the Yellowstone ecosystem.

Calderas are large basin-shaped volcanic depressions that are the remnants of explosive volcanicactivity.

Stratigraphic units of the Yellowstone Plateau volcanic field Volcanic

CyclePrecaldera

RhyoliteCaldera-forming

ash-flow tuff Postcaldera

rhyoliteContemporaneous plateau-marginal basalts

Third Plateau Rhyolite

Basalts of Snake River GroupOsprey BasaltMadison River BasaltBasalt of Geode Creek Swan Lake Flat Basalt

Basalt of Mariposa LakeLava Creek Tuff

(0.64 Ma)

Mount JacksonRhyoliteLewis CanyonRhyolite

Undine FallsBasaltBasalt of WarmRiverBasalt of Shotgun Valley

Second Island Park Rhyolite Basalt of the Narrows

Mesa Falls Tuff (1.3 Ma)

Big Bend RidgeRhyolite

First Big Bend Ridge RhyoliteHuckleberryRidge Tuff

(2.2-2.1 Ma)Rhyolite of Snake River

Butte

Junction Butte Basalt

Table Courtesy of USGS; Volcanic History of Yellowstone Plateau Volcanic Field

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Yellowstone’s Volcanic Past

The first and largest caldera, whichblew up about 2.1 million years ago,was centered in western YellowstoneNational Park but extended intoIsland Park, Idaho (See figure B,next page) . The volume of rock produced by this eruption, known asthe Huckleberry Ridge, producedapproximately 600 cubic miles of rock and ash. In comparison to morerecent volcanic activity, this is about17 times the rock produced duringthe eruption of Mount Tambora inIndonesia, witnessed in April, 1815(See figure A., next page). As aresult of the tremendous amount of ash projected into the stratosphereduring this eruption, meantemperatures dropped from 0.5° tomore than 1° F. Farmers in theUnited States and Europe referred tothis year as the “year without asummer.”

Mount Saint Helens, whicherupted in 1980, produced ¼ of a cubic mile of rock, making the Huckleberry Ridgeeruption 2,400 times larger!

The second eruption occurred 1.3 million years ago, forming the Island Park Caldera. Thesmallest of the three eruptions, this caldera overlays the western part of the Huckleberry Ridgeeruption (See figure C).

The most recent eruption, Lava Creek, erupted just 630,000 years ago. It also overlays theHuckleberry Ridge, and expands eastward of the original caldera boundary approximately 10miles. Because of its relatively young age, the Lava Creek lava flows can be readily viewed andstudied. Yellowstone Lake, the world's largest alpine lake, occupies a portion of the caldera,Yellowstone II, created during the Lava Creek eruption.

Smaller lava flows dating from 150,000 years ago, 110,000 years ago, and 70,000 years ago havefilled in the floor of the Yellowstone II caldera. These smaller flows—composed mostly of rhyolite—have created the topography responsible for Yellowstone's lakes, waterfalls, andstream courses, and have produced the gently rolling terrain characteristic of central Yellowstone.

Tuff is a rock formed as a result of volcanic eruptions and is composedof compacted volcanic fragments; essentially, volcanic ash deposits thathave been turned to stone.

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A.

Reproduced with author permission fromWindows into the Earth: the geologic story of Yellowstone and Grand Teton National Parks byRobert B. Smith and Lee J. Siegel, 2000 Oxford University Press.

B.

Areas of the United States that once were covered by volcanic ash form Yellowstone’sgiant eruptions 2 million and 630,000 years ago, compared with the ashfall from 760,000 year-ago Long Valley caldera at Mammoth Lakes, California, and the 1980 eruption of Mount St.Helens, Washington. (Adapted from Sarna-Wojcicki, 1991)

Reproduced with author permission fromWindows into the Earth: the geologic story of Yellowstone and Grand Teton National Parks by Robert B. Smith andLee J. Siegel, 2000 Oxford University Press.

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C.

Yellowstone’s main volcanic features include 2 million, 1.3 million, and 630,000-year-old calderas as well as Mallard Lake and Sour Creek resurgent domes.

Reproduced with author permission fromWindows into the Earth: the geologic story of Yellowstone and Grand Teton National Parks by RobertB. Smith and Lee J. Siegel, 2000 Oxford University Press.

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Current Eruption Activity at Yellowstone Caldera

Yellowstone contains over 10,000 thermal features , and between 300 and 500 active geysers .Conservatively more than 55% of the world's geysers reside in Yellowstone National Park. Thepark's thermal features lie in the only essentially-undisturbed geyser basin in the world. UnlikeIceland or New Zealand, few park thermal features have been diverted for human use. Globally,

the second largest geyser field is Dolina Geizerov on Russia’s Kamchatka Peninsula containing200 geysers. New Zealand has approximately 40 active geysers, Chile has 38, and Icelandcontains around 25.

Yellowstone’s thermal features are evidence of a continued heat source not far from the earth'ssurface; recent volcanism is the furnace thatprovides the heat for Yellowstone’s geyser andhot spring activity. Hot springs, geysers,fumaroles, and mudpots are found in regions of young volcanic activity. Surface water

deposited as snow on the Yellowstone Plateaupercolates through the cracks and fissures in theearth's surface created by the uplift andexplosion of calderas. This cold water comesinto contact with the shallow magma chambersbeneath the surface, allowing water temperatureto quickly super-heat beyond the boiling point.Because of the great pressure exerted fromwater and rock above, water cannot escape assteam and remains in a liquid state, often attemperatures exceeding 400º F!

The superheated water is less dense than the colderwater sinking around it, and convection currentsdevelop. The more buoyant water is carried towardthe surface of the thermal feature, following thecracks and fissures in the earth's crust. The hot fluids interact with the surrounding rock as they movethrough the underground plumbing system .

Diagram (ABOVE): Yellowstone geyser system. Heat from cooling magma rises to heat water circulating in the porous rock layer. Heat is transferred to groundwater, recharged by rainfall and snowmelt. Shallow hot water canbe trapped and pressurized in small underground reservoirs, released through narrow constrictions, creatinggeysers. Reproduced with author permission Windows into the Earth. Robert B. Smith and Lee J. Seigel. Oxford University Press. 2000.

Much of the composition of the earth's crust beneath Yellowstone consists of rhyolite and basalt .Rhyolite is an important component of many of the geysers we see because it contains anabundance of the element silica (Si) or quartz (SiO 2)—the mineral used to make glass. As hotwater travels through cracks and fissures in the rhyolite, some of the silica is dissolved in thewater and is deposited at the surface in geysers and thermal pools as a rock called siliceous sinter or geyserite. Siliceous sinter also forms a water-tight and pressure-tight lining as a coating in theplumbing systems of Yellowstone thermal features. The pressure tight seal, readily availablewater, and a heat source from the shallow magma chamber are all prerequisites for Yellowstonethermal activity.

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If the movement of the water through theplumbing system is slow enough that pressureis released gradually as water reaches thesurface, a hot spring develops. If there is aconstriction along the plumbing system, ageyser is the more likely outcome. Expandingsteam bubbles build up behind theconstrictions, squeeze through the narrowareas heating the water above, and force thewater to overflow at the surface. The releaseof water at the surface causes a sudden declinein pressure of the super-heated water below

lowering the boiling temperature of the water and allowing steam to form. Water expands rapidlyas steam—to over 1,500 times its original liquid volume— exploding as the geyser erupts. Theeruption continues until water is depleted or the temperature drops below boiling. Once theeruption has ended, the entire process of filling, heating, and boiling, begins again.

Old Faithful Geyser erupting, UpperGeyser Basin, Yellowstone NationalPark.

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Other Thermal Features:

While most of Yellowstone isunderlain by silica-rich rhyolite,the northwest corner of Yellowstone where MammothHot Springs is located has adifferent underlying geology.Approximately 340 million yearsago, sediments rich in calciumcarbonate (CaCO 3) weredeposited in the vicinity of Yellowstone. Geologistsestimate that the waters of Mammoth Hot Springs carrymore than 2 tons of dissolvedCaCO 3 to the surface at a flowrate of 500 gallons per minute.

Water and CO 2 combine to forma weak carbonic acid. Carbonicacid dissolves the CaCO 3 -richlimestone into solution,precipitating travertine limestoneat the surface as temperaturesdecrease. Travertine depositionis rapid—about 8 inches a yearare deposited at Mammoth.

It is thought that thermal watersreach this area of the park in partfrom a fault that runs fromNorris to Mammoth, and fromsurface waters that follow cracksand fissures and are heated frombelow. Geysers do not occur atMammoth for two reasons:travertine is not pressureresistant and could not withstandthe force of an eruption; andwater temperatures don't reachthe boiling point (~93C ° inYellowstone). Thermal waters atMammoth can reach up to 75C°;bubbling that you see is due togas release, not boilingtemperatures.

Mammoth Hot Springs Terraces

Travertine Limestone deposited in TerraceFormations, Mammoth Hot Springs, YNP